The production of oxygen using vacuum swing adsorption (VSA) is well-known to air separation technologists. VSA offers a simple non-cryogenic method to produce gaseous oxygen at purities of 80% to 95%. In the last 20 years oxygen VSA plants have become widespread and are offered in various bed configurations. The multi-bed VSA is typically used in the size rage of 60 tons per day (TPD) and higher. The single bed process was adopted as a lower capital, simpler process for lower production ranges, typically 1 TPD up to 40 TPD. Typical single bed systems usually consist of a single blower train that is used for both the feed air provider as well as the regeneration vacuum system. The process usually incorporates automatic valves to direct the air and vacuum flows during the cycle. A newer embodiment of the single bed process uses a reversing blower to generate the feed stream and apply vacuum for the regeneration step. This latest embodiment is well suited for small to medium sized oxygen VSA production plants (1 to 10 TPD). One example of a single bed reversing blower (SBRB) VSA process of this type is described in U.S. Pat. No. 8,496,738, incorporated herein by reference in its entirety.
Although the single bed reversing blower (SBRB) VSA process is simple in practice, its simplicity comes with performance trade-offs when compared to multi-bed systems. Firstly, the lack of additional adsorber beds does not allow for a crucial bed to bed equalization. The pressure equalization step is key to lowering power consumption and increasing product oxygen recovery. Technologists in the art have overcome this deficiency by adding an equalization tank to the SBRB system (such as equalization tanks in SBRB systems provided by Air Liquide of Houston, Tex.).
Another problem encountered with single bed reversing blower VSA systems is that they are limited to relatively small optimal size, typically being that of small to medium size production plants in the 1 to 40 TPD range. This size limitation is due to the availability of blowers of exceptionally large sizes not being present in the market and lower power efficiency of the VSA process when compared to other processes in the larger (greater than 40 TPD) range.
Larger VSA plants tend to be of the two bed, two blower configuration. These plants are typically field erected and require buildings for the rotating equipment as well as significant civil engineering infrastructure, not only for the buildings but also for cooling water, instrument air, waste process water drains, etc. The typical reliability of a well engineered two bed VSA plant is ninety-eight percent. Reliability is defined as the on-stream availability of the equipment. In most applications this requires the customer to install a liquid oxygen backup system, with additional cost for the civil engineering work for foundations etc. and interconnection into the system, as well as the cost of the liquid oxygen tank. A need exists for improved methods for enhancing the reliability of VSA systems, such as to eliminate or minimize the need for liquid oxygen backup systems and to otherwise more effectively achieve the goals of the customer.